Digital multiplex systems are well-known. Such systems are also known as subscriber loop multiplexers, remote subscriber multiplex terminals or remote line switches. These systems provide carrier communication service between a central office and a group of subscribers associated with one or more remote terminals, the central office being coupled to the terminals over a multiple channel line. For example, my U.S. Pat. Nos. 4,001,514 issued Jan. 4, 1977 and 4,021,618 show one such basic system using a 32 channel line with one form of delta modulation as the coding language.
In systems of this type, a call from the central office is detected in the C.O. terminal unit line circuits, these being a line circuit in the C.O. for each line being served. The call signals are digitized in the line circuit for transmission over the span line during a channel time slot allocated by a concentrator.
In the system shown by the cited references, separate line units each are employed receptive of a particular type of ring signal. Input ring signals are coded and transmitted in the specific code to a line unit receptive of ring signals of the particular type. In the line unit, the signals are decoded and cause a ring output of the designated type to be transmitted.